The present invention relates to a device for the coulometric measurement of the thickness of thin metal layers of different material on a base, with switch-over devices permitting switching to the measurement problem at hand, with a cell switch-over device and with networks assigned to the various measurement problems.
With such measuring instruments, the thickness of thin metal layers can be measured as follows: A cell filled with suitable electrolyte is placed on the layer, the layer to be measured is connected as anode, a cathode is provided, producing an element which has a certain element potential. This element potential has various levels which can be easily distinguished when the right electrolyte is being used. Without current flow, a low potential results. If current flows, a higher potential develops while the layer to be measured is removed and an even higher potential suddenly appears when the layer is finally removed entirely.
This sudden potential increase is commonly used to stop a clock counter for measuring the time which was necessary to remove the metal layer under the action of the constant DC current through the cell. The final indication of this counter may be calibrated in units of the thickness of the layer.
In practice, a large number of measurement problems are of interest. For example, zinc layers, copper layers, silver layers, gold layers, chromium layers, etc. are to be measured. Care must be taken that, before the actual measuring process, a stirring apparatus is set into motion which makes the electrolyte circulate. This is necessary in order to remove air bubbles from the walls of the anode or the cathode. In addition, the stirring apparatus must be turned off at the right moment. One must also determine from what potential threshold the counting process must start and at what potential threshold the counting process must stop. Depending on the situation, different removal currents are necessary. In the digital display of the measured value, the decimal point must appear at the correct location, which requires different networks from case to case. Any case requires a definite pulse sequence as a measure of the removal speed etc. In addition to the variable parameters mentioned so far, there are also constant parameters which are realized by constant electrical networks.
Suitable electro chemical cells with thickness measurement capabilities as well as the electrical circuitry for the voltage supply, regulation circuits to keep the cell current constant, timing circuits, converter circuits for a digital display of the thickness of the metal layer, and so on are wellknown in the art of coulometric thickness measurement devices and therefore not the basic object of the present invention.
Even if it is always the same procedure, which is carried out by the device, when the thicknesses of layers of different metals are to be measured, there is with respect to the different metal layers to be measured a great variety of parameters which require the device to be adaptable to a lot of different sets of such parameters. Some of these parameters are the following:
1. Interface potential at the metal/electrolyte boundry which potential must be overcome by the applied voltage.
2. Different ion mobilities of different sorts of metals.
3. Different valencies of the metal ions.
4. Geometric dimensions of the metal layer and/or of the electro chemical cell.
5. Composition of the electrolyte.
6. Temperature.
In operation, the electrodes of the cell which is selected and used for measurement must be electrically connected to the power supply and the current regulating and signal detecting circuits which are conditioned for proper measurement by adjusting their electrical characteristics in accordance with the above mentioned parameters. To this object, adjustable elements for these cells and circuits are probided in a very usual manner with push-buttons rotatable knobs or the like for manual adjustment. push-buttons,
With devices of this type hitherto known in the art, the circuits were permanently wired inside the device and on the outside of the device there were provided pushbuttons or rotary switches which had to be set for a certain measurement problem. If, e.g., the thickness of copper layers was to be measured, the "Cu" pushbutton was pushed or a rotary switch was set into the associate position.
It is obvious that these devices are relatively inflexible, because in no case can the circuits be changed quickly and inexpensively or can be adapted to unusual measurement problems.
On the other hand, there also may be users who do not wish to make use of all the capacity wired into these instruments, such as, e.g., chromium, nickel, copper, brass, zinc, cadmium, tin, silver, gold.
Finally, repair is expensive and cumbersome if a design element in the networks should fail.
It is, therefore, an object of the present invention to create a device which can be flexibly and inexpensively adapted to any measurement problem, whether it be a standard measurement problem or a more unusual measurement problem.